The present embodiments relate to a method for determining a set of B1 field maps for different transmit channels of a radio-frequency transmit coil of a magnetic resonance apparatus.
Magnetic resonance imaging and the basic principles thereof are already widely known in the prior art. The process entails introducing an object that is to be examined into a relatively high basic magnetic field (e.g., the B0 field). In order to be able to acquire magnetic resonance data, in a slice, for example, the spins of the slice are excited, and the decay of the excitation, for example, is considered as a signal. A gradient coil arrangement may be used to generate gradient fields, while high-frequency excitation pulses (e.g., radio-frequency pulses) are transmitted via a radio-frequency transmit coil. A radio-frequency field (e.g., a B1 field) is generated by the radio-frequency pulses, and the spins of resonantly excited nuclei, spatially resolved by the gradients, are tilted through a flip angle relative to the magnetic field lines of the basic magnetic field. The excited spins of the nuclei then emit radio-frequency signals that may be picked up by suitable receive antennas and processed further in order thus to enable magnetic resonance image data to be reconstructed.
Conventional radio-frequency transmit coils are operated in a “homogeneous mode” (e.g., in a “CP mode”). A single radio-frequency pulse having a defined fixed phase and amplitude is applied to all the components of the transmit coil (e.g., all the transmit rods of a birdcage antenna). In the interests of increasing flexibility and creating new degrees of freedom with the aim of improving the imaging, a technique known as parallel transmission, in which individual pulses that may be different from one another are applied to each of a plurality of transmit channels, is enabled. This totality of individual pulses, which may be described, for example, via the parameters phase and amplitude, is defined in entirety in an activation sequence that is described by a corresponding parameter set. Such a multichannel pulse that is composed of individual pulses for the different transmit channels may be referred to as a “pTX pulse” (e.g., “parallel transmission”). In addition to the generation of location-selective excitations, field inhomogeneities may also be compensated in the process (e.g., as part of “RF shimming”).
In order to determine an activation parameter set of an activation sequence, the effects of the individual transmit channels in the imaging region (e.g., the homogeneity volume) are to be known. These are determined using a technique called “B1 mapping.” In B1 mapping, B1 field maps are acquired for each transmit channel, which provides that the B1 field maps reveal how strong the B1 field is at a specific excitation (e.g., at a unit excitation and/or at a specific transmitter voltage), at a specific location in the imaging region. This provides that each voxel (e.g., volumetric picture element) is assigned a mostly complex B1 value and consequently a B1 amplitude and a B1 phase. The B1 field maps are strongly object-dependent (e.g., at field strengths of the basic magnetic field such as ≧3 T), so that the B1 field maps are acquired individually for each object that is to be scanned. B1 mapping scans may take a very long time in comparison with conventional imaging methods.
Known B1 mapping methods may measure the flip angle caused by a radio-frequency pulse and a phase. The amplitude of the B1 field may easily be determined from the flip angle. The problem here is that all B1 mapping methods have only a limited sensitivity range insofar as the flip angle as a measurement parameter is concerned. The sensitivity range is mostly specified as a region in which the measurement parameter (e.g., the flip angle) may be reliably measured. The sensitivity range is composed of intrinsic limitations inherent in the measurement methods and/or the acquisition technique. Such a limitation in the case of B1 mapping methods may be the fact that only flip angles between 0° and 180° may be resolved, and regions in which an excessively high level of uncertainty and consequently an excessively high error value is present. High error values are produced, for example, as a result of signal noise, such that, for example, in the acquisition techniques used in the B1 mapping method, very small flip angles may be detected only with very great difficulty on account of the signal noise.
Thus, it is known, for example, that signal noise may lead to a systematic overestimation of very small flip angles.
For example, for systems having a plurality of local transmitters, the B1 variation over the object that is to be scanned may be very great. This provides that the transmitter voltage used, a parameter that may be used in selecting the sensitivity range, may not be chosen such that the generated flip angle lies over the object that is to be imaged within the sensitivity range of a scan acquired using the B1 mapping method.
With regard to this problem (e.g., when the dynamic range of the B1 field distribution over the object that is to be imaged is greater than the sensitivity range of the B1 mapping method used), two basic approaches to a solution are known.
It has been proposed to reduce the dynamic ranges of B1 field distributions that are to be acquired. This may be achieved by measuring, not the field distributions of the individual channels and consequently the individual coil elements, but the field distributions of different channel and therefore coil element combinations. The field distributions of the individual channels are to be unequivocally calculated from the measurement results for the transmit channel combinations, ideally with reduced noise sensitivity in comparison with the measurement of the individual transmit channels in each case. The result is therefore a complex backward calculation from a plurality of combinations that are highly susceptible to error and computationally intensive. A greater number of individual B1 field distributions may thus be measured.
In another solution, it has been proposed to cover the dynamic range of the B1 field distributions sequentially in a plurality of measurements. This provides that a plurality of B1 measurements are performed repeatedly for each transmit channel at a different transmitter voltage, the different transmitter voltages generating B1 fields of different strength. A B1 field that falls within the sensitivity range of the B1 mapping method used is to be generated at least once for each subvolume of the imaging region. In this case, therefore, the results of a plurality of measurements using the same acquisition technique, but at different transmitter voltages, are combined in order to obtain the B1 field maps. There is an obvious increase in the measurement and evaluation overhead. The transmitter voltage may also only be varied within certain limits, with the result that no improvement is achieved in relation to the basic problem (e.g., the high signal noise at low flip angles).
Absolute values (or absolute values normalized to a specific transmitter voltage value and a specific excitation) of the B1 field are used for planning the acquisition session and for determining activation parameter sets for activation sequences. Accordingly, although acquisition techniques in which relative B1 field maps may be generated are known, these have the disadvantage that the acquisition techniques do not directly specify the B1 field in terms of the amplitude, but are weighted with a spatial function. The term “relative” in this context refers to the cited weighting function, unknown within the framework of these measurements, which may correspond, for example, to the root of the sum of squares of the amplitudes over all transmit channels. The basic principle of such relative measurement methods is explained in more detail, for example, in DE 10 2005 049 229 B3.
In order to determine the spatial weighting function, reliable B1 field values are to be available at the same picture element both for the absolute B1 amplitude and for the relative B1 amplitude. One possibility of back-calculating the absolute B1 values from relative B1 values is described, for example, in the article titled “Calibration Tools for RF Shim at Very High Field with Multiple Element RF Coils: from Ultra Fast Local Relative Phase to Absolute Magnitude B1+Mapping” by P. F. Van de Moortele et al., Proc. Intl. Soc. Mag. Reson. Med. 15 (2007) 1676. There, it is proposed to perform a further absolute B1 measurement while using all of the transmit channels in order then to be able to solve a linear equation system. Problems of the sensitivity ranges are not addressed therein, however.